2.0 Analysis 2.1 Evidence of an Engine Power Loss Wreckage examination and evaluation established that the engine was likely operating at reduced power prior to the rotor striking the tree. Except for the damage to the transmission support strut which was due to vertical impact forces, there was no deformation of the transmission deck support structure. The absence of damage to the transmission deck and support structure indicates that the power delivered to the main rotor blades when they struck the tree was minimal. Examination of the forward and aft flexible coupling on the engine-to-transmission drive shaft did not reveal any distortion or material failure. This finding supports the conclusion that when the main rotor struck the tree and slowed down, there was no sudden increase in engine power which would normally result as the engine accelerates to compensate for the loss of rotor speed. Had the engine responded to the loss of rpm on striking the tree with a sudden torque increase, the torsional load would probably have caused deformation of the flexible coupling. 2.2 Loss of Engine Power The pilot reported that the aircraft experienced a partial loss of power which required him to lower the collective in order to maintain Nr. The descent angle which resulted was steep and the aircraft rotor blades contacted a tree at the edge of the clearing, the pilot's intended landing spot. When the engine was test run it operated within normal specifications. No evidence as to the cause of the partial power loss was established. 2.3 Flight in Snow Engine power loss as a result of snow ingestion was considered and rejected as a probable cause of the loss of engine power. Historically, if falling snow presents a problem, it will result in a complete loss of engine power (flame-out), not a partial and prolonged decrease in power. Given that the engine did not flame out and that there was no evidence of snow or ice accumulation on the aircraft, it is unlikely that the partial power loss was the result of ingestion of snow by the engine. 3.0 Conclusions 3.1 Findings The aircraft was certified, equipped, and maintained in accordance with existing regulations and approved procedures. The weight and centre of gravity were within the prescribed limits. There was no evidence found of any airframe or engine defect or failure prior to or during the flight. The engine power was low when the main rotor blades struck the tree. The engine lost power for undetermined reasons prior to impact. The pilot was certified, trained, and qualified for the flight in accordance with existing regulations. 3.2 Causes It was determined that there was a partial loss of engine power; the cause of the loss of power was not established. The Board has no aviation safety recommendations to issue at this time.4.0 Safety Action The Board has no aviation safety recommendations to issue at this time.